The present invention is directed to heat exchanger-evaporator devices useful for both liquid and gases, and more particularly is directed to a tube and tube sheet assembly for a heat exchanger device which substantially reduces the amount of turbulence in fluids passing therethrough resulting in reduced wear and erosion of the tube sheet and the tube entrance, thereby prolonging the useful life of the heat exchange device.
In many chemical processes using an evaporator-exchanger, significant erosion occurs at the tube entrance, both within the tubes and the tube sheets supporting the tubes. For example, in a superphosphoric acid evaporator, buildup of scale is known to occur on the tube sheet and tube entrance. This scale buildup occurs as a result of flashing of the superphosphoric acid at the tube entrance which is caused, in part, by the pressure drop and turbulence occurring at the entrance of the evaporator. The scale buildup can be significant, and requires the evaporator to be shut down for descaling of the buildup by either flushing a different solution through the evaporator-exchanger, such as a dilute sulfuric acid, or disassembling the evaporator-exchanger for manually removing the scale.
It is apparent that these descaling procedures interrupt the continuous use of the evaporator and usually remove additional metal from the tube and tube sheet assembly, thereby hastening the mechanical failure thereof. Many attempts have been made to reduce the turbulence of fluids passing through a heat exchanger or evaporator which include the removal of discontinuities occurring in the tube and tube sheet assembly, such as cutting the tubes so that the ends thereof are flush with the tube sheet, and using inserts, which may be formed of plastic, positioned in the openings of the tubes at the tube sheet face, in order to provide a smooth "trumpet" like entry into the tubes. Such inserts have been used to reduce the velocity change from outside the tube sheet to inside the tubes themselves. These inserts, however, may incite further turbulence. Further, they are subject to being dislodged and thereafter blocking the flow of fluid through portions of the heat exchanger, thereby disrupting the flow therethrough and reducing the efficiency of the heat exchanger.
Mechanical failure of an evaporator may be primarily caused by erosion of the tube and tube sheet assembly. Even with very corrosive materials such as superphosphoric acid, the amount of corrosion occurring in an evaporator-exchanger constructed from a corrosion-resistant metal, such as Hastalloy G3, has been found to be only 20% of the tube metal loss. The majority of tube metal loss is caused by entry velocity erosion and removal of the metal during descaling operations.
The present invention enables a liquid or gas to enter the tubes of an evaporator, or heat exchanger, with minimal turbulence, thereby reducing wear and erosion of the heat exchanger tubes at the entrance in the tube exchanger sheet, as well as the tube sheet itself. Since the entry pressure drop of the present invention may be 1/10 of the drop experienced by existing tube and tube sheet assemblies, there is a significant reduction in flashing occurring in the tube entrance which significantly reduces the amount of deposit or scale formation and enables an increase of up to 80% in operating time during an operating cycle because descaling operations are reduced. Because the operating time of the heat exchanger or evaporator is significantly increased, the overall operating cost is reduced. Further, the present invention enables the tube and tube sheet assembly to be cooled proximate the tube sheet to further reduce flashing of entering fluid, thereby additionally reducing fluid flashing.